This is THE source for most military and government decorations, coats of arms and more. You’ll come here if you describe what your soldiers wear or what symbol your sailors sail under. From the website:

The Institute of Heraldry has a long and distinguished record of support to the United States Army. Its roots were firmly planted in 1919 when President Woodrow Wilson directed the creation of the Heraldic Program Office under the War Department General Staff. Its purpose was to take responsibility for the coordination and approval of coats of arms and other insignia for Army organizations. By the end of World War II, its role expanded to include the other military services. In 1957 Public Law 85-263 directed the Secretary of the Army to furnish heraldic services to all branches of the federal government. The Institute’s wide range of heraldic services include decorations, flags, streamers, agency seals, coats of arms, badges, and other forms of official emblems and insignia.

Federal Government – Presidential, Vice-Presidential seals and items from the Director of National Intelligence.

ROTC – ROTC Regulations, Sr. ROTC, and Jr ROTC

If you have sailors in your story, you’ll want to visit the U.S. Naval Ships Coat of Arms section of the site under the Navy. Each Coats of Arms page has a description of the Coat of Arms Elements and a discussion of the symbolism on the Coats of arms, as well as an image of the Coats of Arms themselves.

The FAQ section is worth visiting as well. It is divided into US Flag Etiquette; Army Flags, Guidons, Etc; Organizational Insignia; Insignia and Decorations; and general information.

If you want to use any of the current badges, insignia, decorations and medals in your book, you will need to have written permission from the Institute per their image use policy:

PLEASE NOTE: The images of all badges, insignia, decorations and medals on this web site are protected by Title 18, United States Code, Section 704 and the Code of Federal Regulations (32 CFR, Part 507). Permission to use these images for commercial purposes must be obtained from The Institute of Heraldry prior to their use.